1. Field of the Invention
The present invention relates to a benzobisthiazole compound and a benzobisthiazole polymer. In addition, the present invention also relates to an organic film including the benzobisthiazole compound, the benzobisthiazole polymer or a polymer obtained from the benzobisthiazole compound. Further, the present invention also relates to a transistor including the organic film.
2. Discussion of the Background
Recently, research and development concerning organic electronic devices using an organic semiconductor material have been actively made. Organic semiconductor materials have an advantage over inorganic semiconductor materials such that a thin film can be easily formed using a simple wet method such as printing methods and spin coating methods, and thereby the process for preparing a thin film transistor using an organic semiconductor material can be performed at a relatively low temperature compared to those in the cases of thin film transistors using an inorganic semiconductor material. Therefore, it becomes possible to form such an organic thin film even on a plastic substrate, which has a relatively low heat resistance compared to those of inorganic substrates. Accordingly, light-weight and low-cost electronic devices (such as displays) can be provided by using such an organic thin film. In addition, flexible electronic devices can be provided by using such an organic thin film. Thus, the applications of the organic thin films are broadening.
Various organic thin film transistors using a low molecular weight material or a polymer material have been disclosed. With respect to organic thin film transistors using a low molecular weight material, the following materials have been disclosed:    (1) Acene compounds such as pentacene compounds (Synth. Met., 51, 419, 1992, and published unexamined Japanese application No. (hereinafter JP-A) 05-55568);    (2) Phthalocyanine compounds (Appl. Phys. Lett., 69, 3066, 1996);    (3) Fullerene compounds (JP-A 08-228034 corresponding to UP patent No. 6278127, and Appl. Phys. Lett., 67, 121, 1995);    (4) Anthradithiophene compounds (JP-A 11-195790 corresponding to U.S. Pat. No. 5,936,257);    (5) Thiophene oligomers (JP-A 08-228035 corresponding to U.S. Pat. No. 5,574,291, and Chem. Mate., 4, 457, 1998); and    (6) Bisdithienothiophene (Appl. Phys. Lett., 71, 3871, 1997).
With respect to organic thin film transistors using a polymer material, the following materials have been disclosed:    (1) Polythiophene (Appl. Phys. Lett., 69, 4108, 1996); and    (2) Polythienylenevinylene (Appl. Phys. Lett., 63, 1372, 1993).
However, the films prepared by such low molecular weight materials as mentioned above have poor stability, and the films prepared by such polymer materials as mentioned above have poor qualities due to low purity. Therefore, it is desired to make improvement thereto.
As mentioned above, acene compounds typified by pentacene have been proposed as organic semiconductor materials, for example, in JP-A 05-55568. It is described therein that organic thin film transistors using pentacene for their organic semiconductor layers have a relatively high mobility (i.e., carrier mobility). However, such acene compounds have very poor solubility in popular solvents. Therefore, when a thin layer of an acene compound is formed for forming an active layer of an organic thin film transistor, a vacuum evaporation method has to be used. Namely, simple methods such as printing and coating cannot be used for forming a thin layer of an acene compound. Therefore, acene compounds do not fulfill the need.
It is described in Science, 2004, 303, 5664, 1644-1646 that one of acene compounds, rubrene, has a high solubility in solvents, and a single crystal of rubrene has a very high mobility. However, a film of rubrene, which is prepared by casting a solution of rubrene, does not have a single crystal structure, and therefore the mobility of the film is not sufficient for organic thin film transistors. In other words, a layer of a single crystal rubrene cannot be prepared by a simple method.
It is described in Synth. Met. 84, 269 (1997) that poly(3-alkylthiophene) is used as polymer organic semiconductor materials. Since a regioselective alkyl group is incorporated in the polymeric organic semiconductor materials, the materials are soluble in solvents, although the solubility is low. Therefore, they manage to prepare a thin film of such a polymeric organic semiconductor material using a simple method such as coating and printing.
On the other hand, in order that organic electronic devices stably operate, the organic semiconductor materials used therefor are required to have good oxidative stability. Although a thin film of poly(3-alkylthiophene) can be prepared by a simple method such as coating and printing, the resultant film tends to be easily oxidized because of having a low ionization potential. Therefore, organic thin film transistors using poly(3-alkylthiophene) for the active layer thereof unstably operate in the air. It is described in Synth. Met. 84, 269 (1997) that thieno[2,3-b]thiophene is incorporated in poly(3-alkylthiophene) in such a manner that the conjugated system of the polymer is cut, to improve the oxidative stability of the polymer. Although the proposed compounds have relatively good oxidative stability, the compounds have drawbacks such as insufficient carrier mobility and ON/OFF ratio due to cut of the conjugated system.
JP-A 2005-206750 discloses organic semiconductor materials having a condensed ring including a hetero atom and transistors using the materials in attempting to impart a good combination of carrier mobility and preservability thereto. However, the organic semiconductor materials have poor solubility in solvents, and therefore such simple methods as mentioned above cannot be used for forming thin layers thereof. Namely, organic semiconductor materials having a good combination of solubility, oxidative stability, carrier mobility and ON/OFF ratio cannot be provided.
In addition, it is described in Adv. Mater., 2007, 19, 4160-4165 and 4438-4442 that organic semiconductor materials having a condensed ring including a hetero atom such as thiazole derivatives (e.g., thiazolothiazole and benzobisthiazole) have good stability in the air because the thiazole skeleton has good resistance to oxygen. Further, it is described therein that by extending the π-conjugated system, donor-acceptor interaction is caused between a thiazole unit serving as an acceptor and a thiophene ring serving as a donor, thereby accelerating intermolecular charge transfer. However, even such thiazole compounds have the following drawbacks:
(1) When the thiazole compounds are low molecular weight compounds, layers having good stability cannot be formed; and
(2) When the thiazole compounds are polymers, the polymers have insufficient qualities in view of purity and variation of polymerization degree and controlling of polydispersity.
Because of these reasons, a need exists for an organic semiconductor material having good combination of solubility, oxidative stability, carrier mobility and ON/OFF ratio.